Some 10,000 years ago, at the beginning of the Holocene geological age, there were only around five million of us on the planet. Although humans had a significant impact on the natural world – by using fires to clear forestry or hunting large mammals to extinction – their effects were localised.

Boy, have things changed. In 1900, there were 1.6 billion of us; by 2000 the global population had shot up to 6.1 billion. Last year, we passed the seven billion mark, and best estimates have us reaching the nine billion mark before 2050.

The sheer number of people has profoundly changed the global landscape, as we convert vast tracts of wild vegetation to agricultural or grazing areas, for example. Fishing on an industrial scale to provide for billions has dramatically altered marine diversity. Individual farmers breeding livestock or keeping chickens, when multiplied by millions, have caused biodiversity changes in which more than 90% of the weight of all terrestrial vertebrates is now made up of humans and the animals we've domesticated. The quest for resources to supply us all with materials and the trappings of life has depleted the forests, polluted rivers and soils and even carved the tops of mountains. And the fuels used by each of us for energy have produced combined emissions that are already altering the planet's climate.

By 2050, it is estimated that we could triple our resource consumption to a whopping 140 billion tonnes of minerals, ores, fossil fuels and biomass per year. Our food requirement alone is expected to double by then.

Is our ever-increasing human population propelling us to our doom? Is there a limit to how many people can be sustained on a finite planet – and, if so, have we already passed it?

Grim predictions

It’s not the first time we’ve been presented with this doomsday scenario. More than two centuries ago, when the global population was around an estimated one billion, the British social economist Thomas Malthus issued dire warnings about the risk of population exceeding resource limits. In 1798, he advocated limiting family size and postponing marriage. (As one of seven children, he practiced what he preached by only having three of his own.)

Since Malthus, there has been no shortage of economists, environmentalists and demographers predicting humanity's collapse through famine, wars and epidemics, if we don't check our population. Some environmentalists even go as far as to say it is morally wrong to have children at all.

So far, the doomsayers have been proved wrong: tragedy has been averted through better technologies, the invention of artificial fertilisers, improved medicines and other rescuing remedies. Indeed, there are some examples of where population increase has led to resources being better conserved and managed. For example, Machakos in Kenya, whose population rose to 250,000 – with accompanying resource over-exploitation, denuded hillsides and soil erosion – actually improved when its population rose still further. The extra labour available meant hillsides could be restored and soil erosion tempered, and Machakos is now home to 1.5 million people.

However, whether this improvement can be solely attributed to the increase in population, and whether it can be replicated elsewhere, remains debatable. Examples like Machakos are few and far between – outweighed by the far greater number of societies that have collapsed due to unsustainable resource use, driven by overpopulation.

Birth measures

However, around 1968 (global population count: 3 billion), when Paul Ehrlich’s book The Population Bomb warned of mass starvation due to overpopulation, the rate of human population growth peaked – and then declined. The growth rate has dropped 50% over the past 40 years, as the average woman in developing countries (outside of China) now has three kids, rather than six.

Some countries have taken active steps to promote contraceptive use, delayed motherhood and female education, which has been shown to reduce family size. China, the most populous nation, introduced a controversial one-child policy in 1978, which has prevented hundreds of million potential births. However, this kind of social engineering is still not as effective as economic growth in reducing family size. Taiwan, which moved from a developing to developed nation in the past few decades, has seen a slightly bigger reduction in fertility than China over the same period. Interestingly, Taiwan is one of a handful of countries whose governments now have policies in place to boost fertility by creating incentives for larger families.

The problem is that growth is now almost entirely occurring in regions of the world like sub-Saharan Africa, where people are the poorest, infrastructure is worst and environmental degradation is already problematic. And the current demographic shift, in which developed nations have an older average population, whereas those of developed nations are younger, will continue. It means that the native workforce will be insufficient to support rich-nation societies, which will increasingly rely on migrants from the developing world.

Safe future?

That said, it appears that we are already regulating our population to some degree, with some countries, such as Japan, expected to have a smaller population by 2050. As well as improvements in nutrition and healthcare that have seen more people living for longer, the general global trend is towards reduced fertility (fewer people) and economic growth (greater per-person resource use) – as people get richer, they generally have fewer children.

The decline in population growth is not fast enough for many environmentalists, though. They see reducing human numbers as vital for a safe future for humanity.

However, there are plenty of agronomists and economists who believe there is enough to go around. We can feed nine and even ten billion, they insist, so long as we improve agricultural efficiency in places where outdated practices are used.

But with over half of the world's land already used for agriculture, do we really want to plough the rest? In the next four decades we're going to have to produce more food than we have during the last 10,000 years in soils that are degraded, marginal lands and under the difficult conditions of climate change.

While it may be true that we could feed a couple of billion more people, it is also true that people around the world are starving right now – and not from a shortage in global food supply, but because of wars, or crops already hit by climate change, or being too poor to afford food whose price has been artificially raised by commodities trading. And as people become richer and switch from rice or cassava to milk and meats, the problem will only escalate. Resource use is not just determined by population size, but also by how rich people are.

Ultimately, if everyone on Earth were to eat as the average American does, or to live their lifestyle, there is not enough to go around. The only way we can make this work as a species is by being realistic about resource availability – from cropland to water – and tailor our expectations of what constitutes a good life to the limitations imposed by our population.

We will surely have to become less numerous over time and limit our consumption to sustainable levels. It will mean improving efficiency and reducing waste, but also being less greedy.

In that way, we should be able to accommodate not just an extra two billion people, but as many of the other animals and plants we share our planet with.

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